1. Field
The present application relates to an imaging system.
2. Description of Related Art
Night vision cameras are the mainstay of security systems, search and rescue systems, air traffic control systems, as well as in certain astronomical and scientific missions. Night vision cameras are designed to operate in low illumination conditions where both low intensity visible light and near-IR light (both ambient as well as black body radiation) are utilized for image formation. In military and security related applications, due to the harsh and rapidly changing environmental conditions under which cameras need to operate, two separate video displays are provided, one for the visible spectrum and the other for the near infra-red spectrum. The color camera displays the image when the ambient light intensity is high enough, such as when the subject frame is illuminated by an external visible light source (at dawn or dusk). When the ambient light is low, the night vision camera is turned on to view the same subject frame in low light conditions.
A night vision camera includes an image intensifier to amplify the image by several orders with little distortion. Typically, in the image intensifier, the incident low light (lower than 500 μlux) flux is first converted to electron current in a photoconductive layer. This current is further amplified by 104 to 105 times using an appropriate current amplifier. On the back of the current amplifier, a phosphor layer is deposited which emits greenish light in response to the current incident. As the current amplifier amplifies the incident light faithfully, the emitted greenish light will be a replica of the incident low light image. This greenish light may be focused into the object piece of a Black and White (B/W) CCD camera to obtain the final night vision video. Generally, the current intensifier has an upper limit on the total amount of charge it can deliver in its lifetime after which the gain dramatically drops. Therefore, the regions that are exposed to high intensity light quickly lose their ability to provide high gain, leading to noise, black spots, and graininess in the video over continued operation of the camera.
In real world situations, such as in hot pursuit of a suspect in an otherwise low light urban setting, the above night vision camera will work adequately providing excellent video until a bright light source such as a passing car's headlights or a street light comes into view. In such a scenario, the night vision video shows distinct “blooming”, i.e., whitening of a large part of the screen which blankets parts of the image that are otherwise in a low light region for the sensors. It takes a relatively long time for the current intensifier to recover from the blooming. The net loss of information during recovery of the current intensifier and/or during the switching from the night vision camera to the visible camera could be very costly under critical mission conditions. Also, the repeated exposure to blooming causes the current intensifier to degrade quickly, as explained above, much faster than the manufacturer specified lifetime of the device leading to video noise.